Why Do EMF Meters Measure ELF/VLF Differently?

[Curiousphy]

Why do EMF meters measure ELF and VLF in flux density units but RF in power density units? Is it because ELF/VLF have low power density and high flux density, and RF has high power density and low flux density? They are all EM waves, right? what's driving the difference in measurement units? thanks

 

[Merlin3189]

I don't know anything about EMF meters, but I'd hazard a guess at the reason for discriminating between EM waves at VLF (and below) versus those at substantially higher frequencies (you don't specify a region for these.)
My first thought was near field and far field, as you are always in the near field region at VLF and below. But at what most people call RF, say MF and above, you are still going to be within the near field for much of the time.
So, not knowing your context, I wonder whether it is to do with radiation resistance? In common situations any energised man-made conductor is going to be a very small fraction of a wavelength at VLF and below and have a tiny radiation resistance. So unless the currents are very large, there is very little radiation. On the other hand there can be a detectable magnetic field close to the conductor.
Why or how people measure RF power density, I don't know.

 

[sophiecentaur]

So, not knowing your context, I wonder whether it is to do with radiation resistance?

Or perhaps to do with the fact that the impedance will not be free space. The units could perhaps be E field or H field, depending. If it's to do with the actual interference power onto a signal the significance would be a bit specific. Very different from free space interference into an antenna.

 

[tech99]

Why do EMF meters measure ELF and VLF in flux density units but RF in power density units? Is it because ELF/VLF have low power density and high flux density, and RF has high power density and low flux density? They are all EM waves, right? what's driving the difference in measurement units? thanks

The body is only about 2m long, so it is a very inefficient antenna for the long waves associated with ELF and VLF. Therefore it couples very little power out of a passing wave, and the heating effect is small. On the other hand, the body might be exposed to very large electric fields, for instance in the case of a rigger working on a transmitting antenna. Such fields can produce dielectric heating of the body. Very strong magnetic fields are also subject to recommended limits, as adverse effects are thought to occur.
The transmitting antennas for long waves require huge voltages and currents in order to radiate significant power, so the body is more at risk from these fields than from the radiation intensity. As a matter of interest, we have in the UK a national LF broadcast transmitter radiating 500 kW at 198kHz.
The international recommendations from ICNIRP are published at the following URL:
http://www.icnirp.org/cms/upload/publications/ICNIRPemfgdl.pdf